CH4 – Methane

What is CH4?

CH4 – Methane is the second most important anthropogenic greenhouse gas in the Earth atmosphere. In spite of its global lower concentrations than CO2 – Carbon dioxide in the atmosphere, one CH4 molecule has a green-house power 21 times warmer than one CO2 molecule. Therefore, methane accounts for 20% of the ‘enhanced greenhouse effect’.

The XCH4 product (i.e. near-surface sensitive column-averaged dry air mole fractions of CH4) as derived from SCIAMACHY and GOSAT satellite sensors for 3 latitude bands as a function of years. Figure generated by Dr. Michael Buchwitz, University of Bremen (IUP-UB), through the ESA GHG project (Source: http://www.esa-ghg-cci.org/?q=node/116)

About 60% of the CH4 released in the atmosphere is related to human activities. As a result, its concentration has more than doubled since pre-industrial times, reaching 1774 ± 1 ppbv in 2005 (Forster et al., 2007). This is 150% above pre-industrial levels in 1750. The atmospheric life time of CH4 is 9±2 years, making it a good target for climate change mitigation.

From the 1980’s, the increase in CH4 – Methane has been slowing down, reaching a steady state around the year 2000 (Rigby et al., 2008). After a period of relative stagnation in the early 2000 s (+0.5 ± 3.1 ppb yr−1 increase on average for 2000–2006), all measurements (satellites like the figure above, or ground-based) show that atmospheric CH4 concentration has again rapidly increased since 2007 at more than ten times this rate (+6.9 ± 2.7 ppb yr−1 for 2007–2015; Dlugokencky 2016). The atmospheric growth rate of CH4 accelerated to +12.5 ppb in 2014 and +9.9 ppb in 2015, reaching an annual average concentration of 1834 ppb in 2015 (Dlugokencky 2016). Unlike CO2, atmospheric CH4 concentrations are rising faster than at any time in the past two decades.

Investigating the cause(s) of this increase is a very important question currently addressed by several worldwide scientists. Indeed, these changes of atmospheric CH4 over the past decade are still unclear, primarily because of uncertainties in the global CH4 budget (Saunois et al., 2016). Since 2014, they are now approaching the most greenhouse-gas-intensive scenarios as imagined by IPCC.

The identified potential point and local sources are landfills, mud volcanoes and gas leaks. There are very large relative uncertainties on the emissions of each of those. In March 2012, an accident occurred on the Elgin platform in the North Sea. The CH4 emissions estimates, as indicated by the platform owner, were 200,000 m3 per day. This is equivalent to 100 tCH4 day‑1 or 40 ktCH4 yr‑1. Note however that this was an exceptional event and that regular gas leaks are smaller by several orders of magnitude. As for landfill, the current estimate of the total emission is 30-70 Mt ktCH4 yr‑1 (IPCC, 2007). There are several thousand landfills in the world and typical emission from a single site is on the order of 10 ktCH4 yr‑1 (Themelis and Ulloa, 2007).

Mud volcanoes are recognized as a significant geological source of methane, although the total budget estimates widely vary. Mud volcanoes are represented by about 1,950 prominent individuals worldwide (1100 inland or shallow water and the rest submarine) and about 60 to 65 erupt every year. During the quiescent period, the total emission is estimated to be of 200 ktCH4 yr‑1. Even during the quiescent period, the emission of a single source greatly varies. Dimitrov (2003) suggested that the average annual rate of emitted gas for a single mud volcano varies from 0.4×106 to about 3.3×106 m3 during quiescent periods.

The main uncertainties are from wetland and other inland water emissions. Wetland extent emissions could contribute 30-40% on the estimated range for wetland emissions (Saunois et al., 2016).

There is no large sink of carbon at the surface. The sink is mostly in the atmosphere (oxidation of CH4 to CO2), which leads to a slightly negative (or rather negligible) surface flux. Therefore, CH4 plays a key role in the chemical processes occurring in the troposphere through its oxidation by the OH – hydroxyl radical. This sink reaction, contributes to more than 80% of its total loss in the troposphere (Schneising et al., 2007, Razavi et al., 2009). Other minor removal processes include uptake by soil and transport to stratosphere where CH4 is rapidly destroyed.

A likely major driver of the recent rapid rise in global CH4 concentrations is increased biogenic emissions mostly from agriculture. Tropical regions play the most significant role as contributors to the atmospheric growth. Other sources including emissions from the use of fossil fuels have also increased (cf. Global Carbon Project).

Why shall we observe CH4?

Despite the importance of CH4 as greenhouse gas in the Earth atmosphere, large uncertainties still exist on the location and intensity of emission sources (Dlugokencky et al., 2011 and Bousquet et al., 2006). The two major difficulties in reducing uncertainties come from the large variety of diffusive CH4 sources that overlap geographically, and from the destruction of CH4 by the very short-lived OH – hydroxyl radical.

CH4 – Methane is responsible for 20% of the global warming produced by all greenhouse gases so far. For a time horizon of 100 years, CH4 has a Global Warming Potential 28 times larger than CO2.

CH4 also contributes to tropospheric production of O3 – ozone, a pollutant that harms human health and ecosystems. It also leads to production of H2O – water vapour in the stratosphere by chemical reactions, enhancing global warming (Saunois et al., 2016, ESDD; Kirschke et al., 2013; IPCC 2013 5AR; Voulgarakis et al., 2013).

Furthermore, the role of CH4 sinks has to be further explored as a slower destruction of CH4 by OH – hydroxyl radical in the atmosphere could have also contributed to the observed atmospheric changes of the past decade.

A typical CH4 satellite map?

Typical satellite maps of CH4 can be obtained from GOSAT-FTS sensor since 2009 from the SHortWaveInfraRed (SWIR) spectral band. This 2-year map shows very high CH4 amounts over large industrial areas (e.g. China & India) and regions impacted by large wildfires or biomass burnings (e.g. central-Africa).

EUMETSAT-CNES mission IASI, on-board MetOp A-B-C (MetOp A since 19 October 2006), early morning and evening (9:30), total CH4 column from Thermal InfraRed (TIR). These data are produced by the Science and Technology Facilities Council (STFC) Remote Sensing Group (RSG) at the Rutherford Appleton Laboratory (RAL). They can be downloaded here.

The future Dutch TROPOMI mission, on-board Sentinel-5 Precursor as part of the ESA COPERNICUS program, will deliver CH4 product from SWIR, early afternoon here